Compression "broken" on Hartke 2000

I picked up this used rig for $400 (Hartke 2000, 410TP, 4 space SKG box). Two of the dust covers were rattling, so I'm working on finding something to fix that (sounds fine without, but dust probably isn't a good thing to get inside the speaker). In the mean time, I want to fix this compression problem.

When it's switched off, there's no issue. When switched on at any level, it begins to hum after about a minute or two. The hum gets louder until you turn the compresison off. The compression works the whole time, but of course is moot once the hum starts.

Any clues? I think I'm capable of fixing, as long as I know what to look for...

I am still working on replceing the pots for the countor on my 5000, but I have found that I hate the compression on mine a lot.
I got a Boss bass compressor pedal for about 75 buck a long time ago and wouldn't use the compression on the amp past 3 IMOHO.

I understand the need to have everything working as advertised, but don't be surprised if you want a seperate compressor.